1. Field of the Invention
The present invention relates to a collimator lens mainly used for connecting optical paths or the like in the field of optical communication.
2. Description of the Related Art
Technological innovation in the fields of information and communication today is remarkable indeed and, to meet the requirements for higher communication speed and a dramatically increased capacity for the transmission of information in connection with the extensive use of the Internet, optical signals are replacing electric signals as the principal medium of communication. A majority of backbone cables have been replaced by optical cables, but optical cables are not yet extensively developed for connection between backbone cables and individual users, resulting in an unsatisfactory level of overall communication speed. For this reason, further heightening hope is pinned on thorough coverage by optical information and communication networks.
In establishing an optical communication network, many problems which are not experienced with electric signals arise in the connection and branching of the optical path. Where electric signals are to be transmitted, even if the direction of the circuit formed of a conductor varies complexly, the electric signals are propagated with little attenuation, if any. However, if there is an abrupt directional change of the optical path or misalignment, optical signals will leak out of the optical path and their intensity will significantly attenuate.
For optical path connection, an optical fiber is connected to another, and for changing the direction of the optical path, optical signals emitted from an end of an optical fiber are reflected with a mirror to change their optical path, received by an end of another optical fiber, and propagated by that optical fiber. In any case, a light is discharged from an end of an optical fiber, and the discharged light is received by an end of another optical fiber. The light discharged from the end of the sending optical fiber is expanded and, if received by the end of the other optical fiber, it can be only partially received, resulting in a substantial attenuation of optical signals.
Since an end of an optical fiber is small, only about 10 xcexcm in diameter, it is difficult to position the outlet end of an optical fiber exactly opposite an end of another optical fiber. As a consequence, there is a great loss of optical signals in the connecting part between optical fibers.
Collimator lenses are fitted to ends of optical fibers to connect the optical fibers on the discharging side and the receiving side to increase the diameter of the light emitted from the end of an optical fiber, make the light beam parallel between the collimator lenses and focus the parallel light beam on the end of the receiving optical fiber. By fitting collimator lenses to the ends of optical fibers, the diameter of the light beam can be increased to a few hundred xcexcm, and the alignment of the optical path can be made relatively easy. When collimator lenses are to be fitted to the ends of optical fibers, if their connecting end faces are oriented normal to the optical axis, the reflection loss on the end faces will be great. Especially if the end of the optical fiber guiding optical signals from a laser beam source or the like is normal to the optical axis, the light reflected by that end face is returned to the light source and may give rise to resonance at the source.
In an attempt to solve this problem, the end of the optical fiber is inclined by about 8xc2x0 relative to a plane normal to the optical axis, and the reflection from the end face is thereby prevented. The opposite surface of the collimator lens facing the end of the optical fiber is also inclined at substantially the same angle. If the end face of the optical fiber is inclined and the lens face opposite it is inclined by the same angle, these faces will become parallel to each other. If the optical fiber and the lens are equal in refractive index, the optical axis of the optical fiber and the optical axis of the lens will be in substantially the same direction even though there may be a slight discrepancy. However, since there is a slight difference in angle between the end face of the optical fiber and the end face of the lens and there also is a slight difference between them in refractive index, usually the optical axis of the light coming out of the collimator lens is at some angles relative to the optical axis of the optical fiber. Since the angle of the end face of the optical fiber, that of the end face of the lens and the distance between those end faces differ from collimator to collimator, the angle of the optical axis of the light coming out of the collimator lens also differs from collimator to collimator.
When a light emitted from one optical fiber is to be received by another optical fiber by connecting the optical fibers or by changing the direction of the light emitted from the first optical fiber by having it reflected by a mirror, alignment or adjustment between the two optical fibers is needed to bring the optical axes of the collimator lenses into concurrence. As the freedom of a collimator lens exists on six axes (the three axes of x, y and z for aligning the tip positions, the rotational freedom around the x axis, that around the y axis and that around the z axis), this alignment is nothing easy. Furthermore, even though the optical axes of the lenses are aligned, if the optical axes of the optical fibers are not on the same plane, the aligned plurality of collimators will extend three-dimensionally.
Therefore, an object of the present invention is to provide a collimator lens which makes it possible to limit the direction in which the optical axis deviates within a specific plane and to accomplish alignment easily.
A collimator lens according to the invention comprises:
a cylindrical glass tube,
a columnar fiber chip which is fixed within the cylindrical glass tube and has a through-hole along its center axis, and one end of which is an elliptic surface tilted at a predetermined angle to the center axis,
an optical fiber which is inserted from the other end of the fiber chip into its through-hole and whose tip is positioned on the elliptic surface, and
a columnar lens which has at one end an elliptic surface opposite the elliptic surface of the fiber chip substantially in parallel and is fixed in the cylindrical glass tube, wherein the collimator lens has a reference plane parallel to a plane containing a major axis of the elliptic surface at one end of the columnar fiber chip and a center axis of the columnar lens, the reference plane serving as a reference when the collimator lens is to be aligned.
Preferably, the collimator lens should have a surface or side surface formed by cutting off part of the collimator lens along the reference plane. Preferably the surface or side surface should be formed away from the center axis of the columnar lens by 60% or more of the radius r of the columnar lens. Even more preferably the surface or side surface should be formed away from the center axis of the columnar lens by 60% or more of but less than the radius of the columnar lens.
The collimator lens according to the invention can further have a base plate, onto the upper surface of which is fitted the collimator lens, aligned with the surface or side surface.
The collimator lens can have at least one hole formed from the outer circumference of the glass tube toward the columnar lens center axis along a perpendicular from one point on the columnar lens center axis down to the reference plane. Preferably, the depth of the hole or holes from the outer circumference of the columnar lens should be less than 40% of the radius of the columnar lens.
The collimator lens according to the invention can further have a base plate, wherein the collimator lens is fitted by coupling the hole to a stud provided vertically on the top surface of the base plate.
A collimator lens may be manufactured according to the present invention by the following process:
a cylindrical glass tube, a columnar fiber chip having a through-hole along its center axis and a columnar lens are partially cut off by a plane parallel to the center axis to form side surfaces on the sides of the cylindrical glass tube, the columnar fiber chip and the columnar lens,
an optical fiber is inserted into the through-hole along the center axis of the columnar fiber chip from one of the ends of the through-hole,
elliptic surfaces having angles of 3 to 10xc2x0 to the respective center axes of the columnar fiber chip and the columnar lens and being normal to the side surfaces formed on the sides respectively are formed by grinding the other end of the columnar fiber chip and one end of the columnar lens,
the columnar fiber chip and the columnar lens are assembled into the cylindrical glass tube, so that the elliptic surface of the columnar fiber chip and that of the columnar lens are positioned opposite each other, and
the side surface of the cylindrical glass tube, the side surface of the columnar fiber chip and the side surface of the columnar lens are brought together to form on the side a surface serving as a reference plane parallel to a plane containing a major axis of the elliptic surface at the other end of the columnar fiber chip and a center axis of the columnar lens.
Another method for manufacturing a collimator lens according to the present invention is:
a cylindrical glass tube is provided with a hole formed from an outer surface of the glass tube toward a center axis of the glass tube on a side wall of the glass tube, a columnar lens with a hole, from an outer surface of the lens toward a center axis of the lens, the depth of which is less than 40% of a radius of the lens and a columnar fiber chip with an optical fiber inserted into a through-hole along the center axis of the fiber chip from one end of the fiber chip,
elliptic surfaces having angles of 3 to 10xc2x0 to the respective center axes of the columnar fiber chip and the columnar lens are formed by grinding the other end of the columnar fiber chip and one end of the columnar lens, the elliptic surface formed at the one end of the columnar lens being parallel to the hole of the columnar lens,
the columnar fiber chip and the columnar lens are assembled into the cylindrical glass tube, so that the elliptic surface of the columnar fiber chip and that of the columnar lens are positioned opposite each other, and
the hole of the cylindrical glass tube and the hole of the columnar lens are brought together to fix the columnar fiber chip and the columnar lens within the cylindrical glass tube to form on the side wall a reference hole normal to a plane containing a major axis of the elliptic surface at the other end of the columnar fiber chip and a center axis of the columnar lens.